Combustion control for an internal combustion engine is becoming more sophisticated and complex in recent years. For example, an internal combustion engine in widespread use burns an air-fuel mixture in a lean-burn state in a combustion chamber of a cylinder. Fuel efficiency of the internal combustion engine can be improved by maintaining fuel combustion in the lean-burn state.
Stratified charge combustion is one type of combustion in the lean-burn state. Stratified charge combustion is to ignite and burn an air-fuel mixture in a state where a rich air-fuel mixture is distributed in a vicinity of a spark plug while a lean air-fuel mixture is distributed on a periphery of the rich air-fuel mixture. When the air-fuel mixture is distributed as above, an air-fuel ratio in the entire combustion chamber is too lean for the air-fuel mixture to burn but sufficiently rich in the vicinity of the spark plug. While a contribution can be made to improvement of fuel efficiency, stratified charge combustion has a problem that a nitrogen oxide and a black exhaust are readily produced when an air-fuel mixture is burned while being distributed inappropriately.
In order to solve the problem as above, Patent Literature 1 describes a control apparatus which estimates a level of stratified charge combustion representing a distribution tendency of an air-fuel mixture at a combustible air-fuel ratio in the combustion chamber. The control apparatus detects an air-fuel ratio from a combustion gas exhausted from the internal combustion engine during stratified charge combustion and also estimates a level of stratified charge combustion according to a waveform of the detected air-fuel ratio. In addition, the control apparatus adjusts fuel injection timing according to the estimated level of stratified charge combustion and controls the internal combustion engine to maintain an appropriate level of stratified charge combustion.